1. Field of the Invention
The present invention relates to an optical transmission module and a manufacturing method thereof, and particularly relates to an optical transmission module having an optical element with an optical circuit formed therein and optical connection parts adapted to connect optical fibers to the optical element.
2. Description of the Related Art
In recent years, with a rapid progress in optical communications, enhancement of communication speed and transmission capacity has been promoted. Wavelength-Division Multiplexing (WDM), which is a multiplexing technique using lights with various wavelengths and capable of supporting such enhancement, is gaining widespread use in optical communication systems. The WDM requires highly-accurate optical components for enabling multiplexing and demultiplexing of lights with a wavelength interval of 1 nm or less.
As is commonly known in the art of optical communications, various optical elements are used in transmission channels formed by optical fibers (see, for example, Japanese Patent Laid-Open Publication No. 05-264862). Among the optical components, waveguide components are remarkable in that they are mass-producible and applicable to LSI. Especially, expectations for Planar Lightwave Circuit (PLC), which is one of the waveguide components, is high because it can be connected to optical fibers with a low optical transmission loss and can integrate a number of optical functions therein.
A PLC substrate having such a PLC optically connected to optical fibers is therefore used in high-speed and large-capacity optical communications. The PLC substrate is connected to the optical fibers by fiber arrays, and the fiber arrays are connected to the PLC substrate by optical path forming resins.
Conventionally, the optical elements having PLCs have been made of quartz, Pyrex (™), or the like. However, with recent developments for size and weight reduction and easier production, the core and cladding of optical elements having PLCs can be formed by laminating polymer resins with different refractive indexes on a silicon substrate.
FIG. 8 shows a perspective view of a related-art optical transmission module 10, and FIG. 9 shows side and top views of the optical transmission module 10.
The optical transmission module 10 comprises an optical element 30, and fiber arrays 21 and 22 for connecting optical fibers 11, 12 and 13 to the optical element 30.
The fiber array 21 holds the optical fiber 11, an end face of which is exposed from an end face of the fiber array 21 in the direction of an arrow X2. The fiber array 22 holds the optical fibers 12 and 13, an end face of each of which is exposed from an end face of the fiber array 22 in the direction of an arrow X1. The fiber arrays 21 and 22 are typically made of quartz or a glass material. The end face of the fiber array 21 in the direction of the arrow X2 is bonded to the optical element 30 through an optical path forming resin 41 so as to match the end face of the optical fiber 11 and an end face of an optical path formed in the optical element 30. The end face of the fiber array 22 in the direction of the arrow X1 is bonded to the optical element 30 through an optical path forming resin 42 so as to match the end faces of the optical fibers 12 and 13 and end faces of optical paths formed in the optical element 30.
The following describes the optical element 30 in detail.
FIG. 10 shows a perspective view of the optical element 30.
The optical element 30 is adapted to branch an incident light output and output branched lights form a waveguide, comprising a substrate 31, a lower cladding layer 32, a core 33, and an upper cladding layer 34. The substrate 31 is, for example, a silicon (Si) substrate. The lower cladding layer 32, the core 33, the upper cladding layer 34 are resin laminations formed on the substrate 31.
The following describes a manufacturing method of the optical element 30.
First, the lower cladding layer 32 is formed on the substrate 31. The lower cladding layer 32 is typically made of transparent resin such as fluorinated polyimide. The lower cladding layer 32 is formed by, for example, forming a polyamic acid layer on the substrate 31 with a spin-coating method, and imidizing the layer through a heat treatment.
Then, the core 33 is formed on the lower cladding layer 32. The core 33 as a waveguide is made of the same fluorinated polyimide as the lower cladding layer 32 and the upper cladding layer 34. The composition of the resin of the core 33 is adjusted to have a refractive index different from that of the lower cladding layer 32 and the upper cladding layer 34. For example, when the refractive index of the lower cladding layer 32 and the upper cladding layer 34 is n1 and the refractive index of the core 33 is n2, the composition of the resin is adjusted to haven1<n2.Herein, the refractive index n1 and n2 are, for example, n1=1.525, n2=1.531.
For forming the core 33, a polyamic acid layer is formed uniformly on the lower cladding layer 32 with a spin-coating method, and the layer is imidized through a heat treatment to form a transparent resin layer. Then, the transparent resin layer is coated with a patterned resist, and is dry-etched by a RIE (Reactive Ion Etching) machine to have the lower cladding layer 32 exposed. In this process, a part coated with a photoresist remains unetched to keep the transparent resin layer thereunder. Then, the remaining photoresist is removed. In this way, the core 33 with a desired pattern is formed. The thickness of the core 33 is around 9 through 10 μm, which is substantially equal to the diameter of the optical fibers 11, 12 and 13.
Next, the upper cladding layer 34 is formed to cover the upper and side surfaces of the core 33. The upper cladding layer 34 is made of the same fluorinated polyimide as the lower cladding layer 32, and the composition thereof is adjusted to have the refractive index n1, which is the same refractive index as the lower cladding layer 32. The upper cladding layer 34 is formed by, for example, forming a polyamic acid layer with a spin-coating method, and imidizing the layer through a heat treatment.
In this way, the optical element 30 having a waveguide with a desired pattern is formed.
The optical element 30 having polymer resins with different refractive indexes on the silicon substrate cannot have a resin material on the upper, side, and bottom surfaces thereof due to a large influence of the refractive index. If the optical element 30 has the resin material on those surfaces, a light passing through the core 33 is lost. For this reason, the optical element 30 is connected to the fiber arrays 21 and 22 only at the end faces thereof.
However, the end faces of the optical element 30 are small, so that the connection areas between the optical element 30 and the fiber arrays 21 and 22 are also small. This makes connections between the optical element 30 and the fiber arrays 21 and 22 weak. Therefore, the optical transmission module 10 with a silicon substrate described above is disadvantageous in this respect.